Increases in adolescent marijuana and other drug use have created widespread concern.

One theory argues that increased use of cigarettes and alcohol among younger adolescents leads to greater use of marijuana which, in turn, leads to subsequent use of other drugs (e.g. cocaine, heroin, hallucinogens).

Detractors of this theory claim that use of these substances is a symptom of a larger set of destructive behaviors (e.g. violence, suicide, promiscuous sex), and marijuana has no independent effect on the use of other more serious drugs.

The authors examined whether, for high school seniors, early use of cigarettes, alcohol and marijuana has an independent effect on more serious drug use even when other behaviors are considered.

Using the 1995 Youth Risk Behavior Survey (n=2871) and logistic analysis, after accounting for selected other behaviors, seniors using cigarettes before age 13 were 3.3 (95% C.I. 2.3,4.6) times likelier to have used marijuana than ones who never smoked ; for alcohol, the odds ratio was 4.5 (2.6,7.7).

Seniors using marijuana before the age of 14 were 7.4 times (4.0,13.6) likelier to have used other drugs.

Though no causal effect is demonstrated, cigarette and alcohol use was associated with the likelihood of marijuana use ; marijuana use was associated with the likelihood of other drug use, even after selected other risk and protective behaviors were considered.